A rotary encoder is an important sensor commonly used in closed loop velocity control and positioning applications within industrial applications, for example, in plants for paper or steel making in which the rotary encoder may be arranged to detect rotary movement of a machine shaft. In general, rotary encoders include a detector part which detects rotary motion and generates an electric signal corresponding to the detected angular position change. Different values, such as angular velocity and number of revolutions may be derived from the signal. There are several different types of rotary encoders, for example, optoelectrical encoders. This type of encoder includes an encoding disc that has an optically readable pattern. The disc pattern is read by one or more detectors which each deliver an electric signal in relation to the amount of light that is received in the detector, so that movement of the encoding disc in relation to the detector will be indicated by changes in the electric signal. The encoding disc includes a periodic pattern, such as a plurality of light and dark fields of mutually the same size, for instance. When the encoding disc is twisted or rotated, the change between dark and light fields can be detected and a change in angle thus determined.
The detector in the optoelectrical encoder includes a light sensitive semiconductor chip which delivers the electric signal in relation to the amount of light that is received by the light sensitive semiconductor chip. This light sensitive semiconductor chip is often mounted on a circuit board. The light sensitive semiconductor chip often has contact areas on the upper light sensitive side. In order to connect these contact areas to the circuit board on which the light sensitive semiconductor chip is mounted wire bonding is used.
Wire bonding is a common method of making interconnections between semiconductor chips and a circuit board during semiconductor device fabrication. Wire bonding must however be done in a clean room environment. Providing a clean room environment during manufacturing of encoders is expensive.
Encoders are often used in environments that are subjected to much vibration, which may cause the bond wires to come off. Vibrations or shocks may also cause mechanical contact with bond wires. These problems cause the encoder to malfunction. A faulty rotary encoder could lead to unscheduled stops of machines or plants for service or replacement of the encoder. This is disadvantageous since it leads to costly drops in production.
One way to protect against humidity and mechanical contact between the bond wires is to use silicon around the bond wires. The silicon may however move with temperature changes. This can result in that the bond wires and/or the light sensitive semiconductor come loose.
In view of the foregoing discussion, there is believed to be a need for an improved solution for the use of light sensitive semiconductor chips in encoders which solves or at least mitigates at least one of the above mentioned problems.